(1) Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more particularly to an air-fuel ratio control system having an evaporated fuel purge system with a learning and controlling function.
(2) Description of the Related Art
An internal combustion engine is know in the art, which engine has a canister for temporarily storing evaporated fuel and an air-fuel ratio sensor provided in an exhaust line, and wherein an amount of injected fuel is corrected using a feedback correction factor (FAF) in accordance with an output signal from the air-fuel ratio sensor so that the air-fuel ratio is a predetermined target air-fuel ratio.
Additionally, in a case where a suction air amount is detected by means of a pressure in an intake line, a basic fuel-injection amount is determined in accordance with a rotating speed of the engine and an internal pressure of a surge tank measured by an intake pressure sensor. In such a case, it is necessary to correct a basic fuel-injection amount map because of degradation of characteristics of the sensors and actuators (for example, a fuel injector) due to aging. To overcome the degradation due to aging, a control system for determining the basic fuel-injection amount has a function to learn an effect resulting from the degradation due to aging. The effect may degrade the air-fuel controlling operation efficiency. In the control system, learning values are used for eliminating the effect resulting from the degradation due to aging. The learning values may be respectively provided for a plurality of engine condition ranges. These ranges may have been previously obtained by dividing all of the entire engine operating conditions into a plurality of operating condition ranges, which dividing is performed in accordance with the intake line pressures occurring during the operation of the engine. The learning values may be corrected by varying (updating) them according to the degradation of the characteristics in the engine due to aging. This correction of the learning values may be achieved using the variation (shifting) of the FAF from a reference value.
Japanese Laid-Open Patent Application No.6-17716 discloses the above-mentioned kind of air-fuel ratio control system. This system obtains a deviation of the air-fuel ratio by separating it into a deviation (purge deviation) due to a purge effect and a deviation (aging deviation) due to aging effect. The system learns only the aging deviation due to aging so as to apply it to the air-fuel control. This allows to obtain the learning values while performing a purge operation, and thus an accurate learning operation can be performed while maintaining a sufficient purge amount of fuel.
The above-mentioned air-fuel ratio control system is suitable for a four-cylinder engine in which purged evaporated-fuel is uniformly introduced to each cylinder, and is able to perform an accurate learning operation.
However, the above-mentioned air-fuel ratio control system is not suitable for an engine having a V banked cylinder structure, such as a 6-cylinder V type engine, in which cylinders are divided into two groups, for example, a first bank comprising the first, third and fifth cylinders and a second bank comprising the second, fourth and sixth cylinders. In this type of engine, an intake manifold is branched so that a fuel-air mixture is directed to each of the cylinder groups. Accordingly, evaporated fuel is separately purged to an intake line of each cylinder group. The purged amount of evaporated fuel may vary from one cylinder group to another cylinder group due to fluctuation in an intake air amount and fluctuation in an exhaust gas flow generated by an exhaust gas recirculation system (EGR)
For example, if an amount of evaporated fuel purged to the first bank increases, the amount of evaporated fuel purged to the second bank decreases. Accordingly, in this case, the state of the air-fuel ratio in the first bank becomes too rich, while that of the second bank becomes too lean. That is, in the engine having the V banked cylinder structure, improper distribution of evaporated fuel to be purged to the first and second banks also causes variation of the air-fuel ratio.
Accordingly, there is a problem in that the conventional air-fuel control system, if it is applied to the V type engine, may erroneously recognize and learn the above variation of the air-fuel ratio as being the aging variation.